The advance of semiconductor technology not only effectively reduces the size of electronic elements but also obviously decreases the fabrication cost of electronic products. For many years, the semiconductor technology was limited to fabricate planar semiconductor structure via etching, ion implantation, wiring, etc. The smallest chip has been as small as 6F2 so far. However, the technical advance in reducing the feature size has been gradually slowed down, and it is hard to further prominently reduce the area occupied by a chip in a wafer. On the other side, the vertical (also called as three-dimensional) semiconductor technology is growing mature, wherein the semiconductor elements are vertically grown on a wafer to reduce the area occupied by a transistor in the wafer and reduce the chip size to as small as 4F2.
For examples, a U.S. Pat. No. 7,524,725 disclosed a “Vertical Transistor of Semiconductor Device and Method for Forming the Same”, and a U.S. Pat. Pub. No. 2006/0258086 disclosed “Methods of Forming Vertical Transistor Structures”. In the abovementioned prior arts, a silicon substrate is etched to form a plurality of pillars, and ion-implanted regions are formed on the pillars via an ion-doping method or a diffusion method, respectively functioning as the sources, the channels and the drains. Refer to FIG. 1 schematically showing the ion distributions of a source region 1, a channel region 2 and a drain region 3. Limited by the ion-implant technology, the ion concentration 4 of the channel region 2 is not uniform but in a Gaussian distribution along the vertical direction, wherein the ion concentration 4 of the channel region 2 vary in different depth. In such a case, the ion concentration 4 may respectively overlap the ion concentration 5 of the source region and the ion concentration 6 of the drain region and form a high-concentration overlap section 7 and a low-concentration overlap section 8. Such a phenomenon may results in leakage current or conduction failure and affects the quality of vertical transistors.